Local heat treatment method for controlling residual stress with primary and auxiliary heating

Abstract

A primary heating band is applied to a weld to control a microstructure and hardness of the weld and uniformity of the structure to realize micro-control of the residual stress; an auxiliary heating band is applied a certain distance away from the weld to generate compressive stress on an inner surface of the weld to realize macro-control of the compressive stress. Reinforcement with a rib plate is eliminated, and a labor intensity and a construction period are reduced. The method reduces deformation near the weld and transfers the largest deformation to a non-weld zone; by applying the auxiliary heating and strictly controlling a time interval between primary heating and auxiliary heating, the structure is improved and the welding residual stress is controlled at the same time; a local heat treatment effect is optimized, and a small tensile stress or compressive stress is generated on the inner surface of the weld.

Claims

1. A local heat treatment method for controlling residual stress with primary and auxiliary heating, wherein a primary heating band is applied to a weld and an auxiliary heating band is applied a certain distance from the primary heating band, and the method comprises the following steps: I. determination of a heat treatment process of the primary heating band at step 1, preliminarily determining the heat treatment process of the primary heating band, comprising determining key process parameters of heat treatment according to a heat treatment object in combination with respective inherent features and corresponding local heat treatment purposes as well as technical design documents and relevant standards and regulations, wherein the key process parameters comprise a temperature increase and decrease rate, a holding temperature, a holding time, and a width W.sub.PHB of the primary heating band; at step 2, optimizing the heat treatment process of the primary heating band, comprising determining whether a width of a uniform temperature band and temperature uniformity along a thickness direction satisfy requirements based on numerical simulation calculation, and performing verification through a heat treatment simulation test based on the determination to optimize the key process parameters of the primary heating band; II. determination of a heat treatment process of the auxiliary heating band, wherein heat treatment process parameters of the auxiliary heating band comprise a distance W.sub.DCB of a central position of the auxiliary heating band from the primary heating band, a highest temperature T.sub.A of the auxiliary heating band and a width W.sub.AHB of the auxiliary heating band; at step 3, determining the distance W.sub.DCB of the central position of the auxiliary heating band from the primary heating band, comprising building a finite element model to perform welding and heat treatment simulation based on a heat treatment process curve and the key process parameters determined at step 2, and checking a change result of axial stress or transverse stress during a heat treatment temperature holding process to determine a middle position where compressive stress is generated, and the middle position where compressive stress is generated is away from the center of the weld in the following distance: W.sub.PHB<W.sub.DCB<2W.sub.PHB, so that the distance W.sub.DCB of the central position of the auxiliary heating band from the primary heating band is obtained; at step 4, determining the highest temperature T.sub.A of the auxiliary heating band, comprising firstly, assuming that the width of the auxiliary heating band is the width of the primary heating band at the central position of the auxiliary heating band determined at step 3, and then comparing stress distributions after heat treatment at different holding temperatures to determine the highest temperature T.sub.A of the auxiliary heating band; at step 5, determining the width W.sub.AHB of the auxiliary heating band, comprising changing the width of the auxiliary heating band based on step 4 to determine the optimal width of the auxiliary heating band, wherein the width W.sub.AHB of the auxiliary heating band is 0.5W.sub.PHB<W.sub.AHB<W.sub.PHB; III. optimization of local heat treatment processes of primary heating and auxiliary heating at step 6, controlling the residual stress by the primary and auxiliary heating bands, comprising after the heat treatment processes of the primary and auxiliary heating bands are obtained, the impact of the temperature increase time of the auxiliary heating band is studied based on numerical simulation to determine a temperature increasing time of the auxiliary heating band, where the temperature of the auxiliary heating band increases after the temperature of the primary heating band increases; the specific local heat treatment method includes the following: firstly the primary heating band at a weld is heated to the holding temperature, the auxiliary heating band is heated when the primary heating band starts to cool down, and the auxiliary heating band starts to cool down when the primary heating band is cooled down to 100-150° C.

2. The local heat treatment method for controlling residual stress with primary and auxiliary heating according to claim 1, further comprising implementation of heat treatment, which specifically comprises, performing thermocouple spot-welding and laying of heating sheets and temperature holding tooling based on the determined heat treatment scheme, and wire-connecting a temperature measuring thermocouple, a temperature control thermocouple and a compensation thermocouple with a paperless recorder and a temperature control cabin to ensure that the thermocouples are and heat treatment-related equipment are fault-free, and then performing heat treatment.

3. The local heat treatment method for controlling residual stress with primary and auxiliary heating according to claim 1, wherein the primary heating band is applied to the weld to control a microstructure and hardness of the weld and uniformize the structure so as to realize micro-control of the residual stress; the auxiliary heating band is applied a certain distance from the weld to generate compressive stress on an inner surface of the weld so as to realize macro-control of the compressive stress.

4. The local heat treatment method for controlling residual stress with primary and auxiliary heating according to claim 1, wherein the heat treatment object is a large-size reinforcement plate weld, a pipe circumferential weld, a cylinder closure weld, or a flat plate structure on a pressure vessel.

5. The local heat treatment method for controlling residual stress with primary and auxiliary heating according to claim 4, wherein a diameter of an opening on the pressure vessel of the reinforcement plate is greater than or equal to 4 m.

6. The local heat treatment method for controlling residual stress with primary and auxiliary heating according to claim 4, wherein the reinforcement plate is a circular reinforcement plate or a square reinforcement plate; for the circular reinforcement plate, six-segment three-stage symmetrical heat treatment is adopted; for the square reinforcement plate, four-segment two-stage symmetrical heat treatment is adopted.

7. The local heat treatment method for controlling residual stress with primary and auxiliary heating according to claim 4, wherein when a radius-thickness ratio of the closure weld is greater than 500, segmented symmetrical heat treatment is adopted as follows: a whole circumference is divided into several symmetrical segments to perform symmetrical heat treatment.

8. The local heat treatment method for controlling residual stress with primary and auxiliary heating according to claim 1, wherein when the primary heating band adopts induction heating, determination of arrangement of an induction cable based on numerical simulation is further comprised in step 2.

9. The local heat treatment method for controlling residual stress with primary and auxiliary heating according to claim 1, wherein the highest temperature of the auxiliary heating band is 40-60% of the heat treatment holding temperature of the primary heating band in step 4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a principle diagram of primary and auxiliary heating according to an example of the present disclosure.

(2) FIG. 2 is a cloud chart of compressive stress generated by a circular reinforcement plate during temperature holding process according to a first example of the present disclosure.

(3) FIG. 3 is a diagram of segmented arrangement of primary and auxiliary heating along a circular reinforcement plate and a curve chart of axial stress of an inner lower path according to a first example of the present disclosure.

(4) FIG. 4 is curve chart of axial stress and circumferential stress of a vertical cylinder closure weld according to a second example of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) The present disclosure is described in detail below in combination with specific examples.

(6) A local heat treatment method for controlling residual stress with primary and auxiliary heating includes: applying a primary heating band to a weld, and applying an auxiliary heating band a certain distance away from the primary heating band.

(7) The method includes the following steps.

(8) I. Determination of a Heat Treatment Process of the Primary Heating Band

(9) At step 1, the heat treatment process of the primary heating band is preliminarily determined.

(10) Key process parameters of heat treatment are determined according to a heat treatment object in combination with respective inherent features and corresponding local heat treatment purposes as well as technical design documents and relevant standards and regulations. The key process parameters include a temperature increase and decrease rate, a holding temperature, a holding time, and a width (W.sub.PHB) of the heating band.

(11) At step 2, the heat treatment process of the primary heating band is optimized.

(12) Whether a width of a uniform temperature band and temperature uniformity along a thickness direction satisfy requirements is determined based on numerical simulation calculation, and verification is performed through a heat treatment simulation test based on the determination to optimize the key process parameters of the primary heating band.

(13) II. Determination of a Heat Treatment Process of the Auxiliary Heating Band

(14) Heat treatment process parameters of the auxiliary heating band include a distance W.sub.DCB of a central position of the auxiliary heating band from the primary heating band, a highest temperature T.sub.A of the auxiliary heating band and a width W.sub.AHB of the auxiliary heating band.

(15) At step 3, the distance W.sub.DCB of the central position of the auxiliary heating band from the primary heating band is determined.

(16) A finite element model is built to perform welding and heat treatment simulation based on a heat treatment process curve and the key process parameters determined at step 2, and a change result of axial stress (rotary structure) or transverse stress (flat plate structure) during a heat treatment temperature holding process is checked to determine a middle position W.sub.DCB where compressive stress is generated, and the middle position W.sub.DCB where compressive stress is generated is away from the center of the weld in the following distance: W.sub.PHB<W.sub.DCB<2W.sub.PHB, so that the distance W.sub.DCB of the central position of the auxiliary heating band from the primary heating band is obtained.

(17) At step 4, the highest temperature T.sub.A of the auxiliary heating band is determined.

(18) Firstly, it is assumed that the width of the auxiliary heating band is the width of the primary heating band at the central position W.sub.DCB of the auxiliary heating band determined at step 3, and then stress distributions after heat treatment at different holding temperatures are compared to determine the highest temperature of the auxiliary heating band.

(19) At step 5, the width W.sub.AHB of the auxiliary heating band is determined.

(20) The width of the auxiliary heating band is changed based on step 4 to determine the optimal width of the auxiliary heating band. The width W.sub.AHB of the auxiliary heating band is 0.5W.sub.PHB<W.sub.AHB<W.sub.PHB.

(21) III. Optimization of Local Heat Treatment Processes of Primary Heating and Auxiliary Heating

(22) At step 6, the residual stress is controlled by the primary and auxiliary heating bands.

(23) After the heat treatment processes of the primary and auxiliary heating bands are obtained, the impact of the temperature increase time of the auxiliary heating band is studied based on numerical simulation to determine a temperature increasing time of the auxiliary heating band, where the temperature of the auxiliary heating band increases after the temperature of the primary heating band increases.

(24) The specific local heat treatment method includes the following: firstly the primary heating band at the weld is heated to the holding temperature, the auxiliary heating band is heated when the primary heating band starts to cool down, and the auxiliary heating band starts to cool down when the primary heating band is cooled down to 100-150° C.

(25) IV. Implementation of Heat Treatment

(26) At step 7, the heat treatment is implemented.

(27) According to the determined heat treatment solution, thermocouple spot welding, and laying of heating sheets and temperature holding tooling are carried out, and a temperature measuring thermocouple, a temperature control thermocouple, and a compensation thermocouple are wire-connected with a paperless recorder and a temperature control cabin to ensure thermocouples and heat treatment-related devices are fault-free and then heat treatment is carried out.

(28) Further, in an example of the present disclosure, the heat treatment object is a large-size reinforcement plate weld, a pipe circumferential weld, a cylinder closure weld, or a flat plate structure on a pressure vessel.

(29) Further, in an example of the present disclosure, a diameter of an opening on the pressure vessel of the reinforcement plate is greater than or equal to 4 m.

(30) Further, in an example of the present disclosure, the reinforcement plate is a circular reinforcement plate or a square reinforcement plate. For the circular reinforcement plate, six-segment three-stage symmetrical heat treatment is adopted; for the square reinforcement plate, four-segment two-stage symmetrical heat treatment is adopted.

(31) Further, in an example of the present disclosure, when a radius-thickness ratio of the closure weld is greater than 500, segmented symmetrical heat treatment is adopted: a whole circumference is divided into several symmetrical segments to perform the symmetrical heat treatment.

(32) Further, in an example of the present disclosure, when the primary heating band adopts induction heating, determination of arrangement of an induction cable based on numerical simulation is further included in step 2.

(33) Further, in an example of the present disclosure, the highest temperature of the auxiliary heating band is 40-60% of the heat treatment holding temperature of the primary heating band in step 4.

Example 1

(34) As shown in FIGS. 1-3, an ultra-large pressure vessel cylinder is 50 mm in thickness and 40 m in diameter. The reinforcement plate is a “circular” reinforcement plate which is 120 mm in thickness and about 4.2 m in diameter. The width of the primary heating band is 600 mm, a spacing between the primary heating band and the auxiliary heating band is 400 mm, the width of the auxiliary heating band is 400 mm, and the highest temperature of auxiliary heating is 500° C. Numerical simulation is performed according to a heat treatment condition and relevant input. A cloud chart of the stress after temperature increase is obtained by only a primary heating method, i.e. “bull's eye” type heating, as shown in FIG. 2. It may be seen from FIG. 2 that a large compressive stress is generated above and below the reinforcement plate, which is about 600 mm away from the weld. Based on this, the local heat treatment method by the synergy of primary heating and auxiliary heating is performed. FIG. 3(a) is a diagram of segments and arrangement of primary and auxiliary heating. FIG. 3(b) is a path distribution diagram of axial stress of a lower path on an inner surface of the reinforcement plate after the method is adopted. It may be seen from FIG. 3(b) that by adopting the method, an obvious heat treatment effect is achieved and the compressive stress is generated on the inner surface.

Example 2

(35) As shown in FIG. 1, an axisymmetric model is built. A cylinder closure weld is φ20000×50×92000 with a V-shaped groove and a total of 60 welds. The width of the primary heating band is 400 mm, the spacing between primary heating and auxiliary heating is 300 mm, the width of the auxiliary heating band is 300 mm, and the holding temperature of auxiliary heating is 300° C. Welding and heat treatment process analysis are performed for the cylinder closure weld by the numerical simulation method. The local heat treatment method with primary and auxiliary heating is adopted. Distributions of axial stress and circumferential stress of the path P1 near the weld on an inner wall of the cylinder closure weld are output as shown in the drawing. In FIG. 4(a), the ordinate represents the axial stress of the path P1 along the inner wall of the cylinder closure weld, and the abscissa represents that a distance from each side of the weld is 30 mm; in FIG. 4(b), the ordinate represents the circumferential stress of the path P1 along the inner wall of the cylinder closure weld, and the abscissa represents that a distance from each side of the weld is 30 mm. It can be seen from FIG. 4(a) and FIG. 4(b) that the local heat treatment method for controlling residual stress with primary and auxiliary heating in the present disclosure can significantly reduce the axial stress and the circumferential stress near the weld and change a tensile stress into a compressive stress. Compared with the traditional heat treatment method in which heating is performed only near the weld, the local heat treatment method for controlling residual stress with primary and auxiliary heating in the present disclosure can significantly reduce axial and circumferential stress values, achieving the optimal heat treatment effect.

(36) Certainly, the foregoing descriptions are not intended to limit the present disclosure, and the present disclosure is also not limited to the above examples. Changes, modifications, additions or substitutions made by persons skilled in the art within the scope of essence of the present disclosure shall also be encompassed in the scope of protection of the present disclosure.